1. Field of the Invention
The present invention relates to a composite material including a liquid crystal and a polymer.
The invention also relates to a display device using the above composite material.
Further, the invention relates to a process or method of manufacturing the composite material and the display device.
2. Description of the Related Art
In recent years, it has been proposed to use a composite material including a liquid crystal and a polymer as a liquid crystal material, e.g., for displaying characters and graphics. It has been expected that the composite material including a liquid crystal and a polymer can provide a bright liquid crystal panel of a large area, because such composite material does not require a polarizer and manufacturing of a cell.
For example, U.S. Pat. No. 5,437,811 has disclosed that the following composite material exhibits a memory effect. In this composite material, a chiral nematic liquid crystal made of a nematic liquid crystal and a chiral material added thereto is used as a liquid crystal (cholesteric liquid crystal) exhibiting a cholesteric phase, and a slight amount of polymer is added to the cholesteric liquid crystal.
When a low voltage in a pulse-form is applied to a liquid crystal cell, in which such cholesteric liquid crystal and polymer are retained between two transparent plates, through electrically conductive transparent films arranged inside the respective plates, a focal conic orientation, i.e., a phenomenon that helical axes of liquid crystal molecules are irregularly directed occurs, so that the incident light is scattered, resulting in an opaque appearance of the liquid crystal. When a high voltage in a pulse-form is applied, the helical axes of liquid crystal molecules are oriented vertically with respect to the plates and thus form the planar orientation, so that the liquid crystal exhibits a transparent appearance. These two states are maintained stably, and in other words, bistability of these states is maintained even after the voltage application is terminated. It has been considered that the bistability is achieved owing to the fact that the polymer restricts movement of the liquid crystal.
The above composite material requires no memory elements such as TFT and MIM and has been paid attentions to as an element for providing a high-precision display device.
Meanwhile, a cholesteric liquid crystal in the planar orientation selectively reflects light having a wavelength, which corresponds to the product of a helical pitch length and an average refractive index or refractivity of the liquid crystal, and thereby attains a colored transparent state. Therefore, it has been attempted to use the cholesteric liquid crystal for providing a color display.
However, the selective reflection wavelength of the cholesteric liquid crystal sensitively changes depending on a temperature and a viewing angle, and thus it is difficult to maintain the cholesteric liquid crystal at a constant color.
As one of pleasures for overcoming the above problem, such a cholesteric liquid crystal may be used that the selective reflection wavelength is, for example, in the infrared range, as disclosed in U.S. Pat. No. 5,437,811. Thereby, the cholesteric liquid crystal allows transmission of visible light to exhibit a transparent appearance, and it is possible to prevent an influence on light scattering and transmitting characteristics of the liquid crystal in a visible range even when the selective reflection wavelength of the liquid crystal changes due to change of the helical pitch length depending on a temperature and/or the viewing angle of the liquid crystal. Further, it is possible to provide a two-color display such as white and black display by utilizing background color as well as two states, i.e., transparent and opaque states, which can be achieved by adjusting the selective reflection wavelength of the liquid crystal to be in a range (e.g., an infrared range) outside the visible range.
However, when the selective reflection wavelength is set, e.g., in the infrared range, such a problem arises that a contrast decreases in accordance with increase of the helical pitch length. More specifically, the liquid crystal itself generally exists as a flowable material, so that the composite material must have a hardness (form retention property) to an extent allowing retention of the form, for example, in order to facilitate handling of the liquid crystal as a display element. If an addition rate of the polymer is increased for ensuring the form retention, a transmittance or transmission factor of visual rays in the planar orientation or state decreases in accordance with increase of the helical pitch length, resulting in reduction of contrast.